Apparatus for implantation into bone

ABSTRACT

An anchor is implantable into a bone in a patient&#39;s body and, when implanted, is resistant to toggling in the bone and to being pulled from the bone. The anchor includes a head end portion having a surface that extends transverse to a central axis of the anchor and is engagable with the bone. A plurality of helical spikes extend from the surface on the head end portion and are engagable with the bone. Each of the plurality of helical spikes has a helical central axis that forms a helix around the central axis of the anchor. Each of the plurality of helical spikes further has a circular cross-sectional configuration as viewed in a plane extending perpendicular to the helical central axis of each of the helical spikes and a distal end portion with a tip that penetrates the bone as the head end portion is rotated relative to the bone.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/708,940, filed Nov. 8, 2000, which correspondsto U.S. Provisional Patent Application Serial No. 60/238,271, filed Oct.5, 2000, and which is assigned to the assignee of the presentapplication.

TECHNICAL FIELD

[0002] The present invention is directed to an apparatus forimplantation into a bone in a patient's body, and is particularlydirected to an apparatus that, when implanted, is resistant to togglingin the bone and to being pulled from the bone.

BACKGROUND OF THE INVENTION

[0003] Bone screws are used in the medical field for a variety ofpurposes. Typical uses for bone screws, also referred as bone anchors,include treating a bone fracture, attaching a corrective device to partsof a fractured bone in an area adjacent to the fracture, and attaching acorrective device to a group of bones, such as vertebrae of a spinalcolumn.

[0004] Most known bone screws use a conventional screw design, i.e. asolid shank, with one or more external thread convolutions. The solidshank and external threads of the conventional bone screws can cause thebone screws to displace an undesirably large amount of bone whenimplanted. It is also known to use a corkscrew-style helical spike as atissue anchor. The known corkscrew-style tissue anchors, when implanted,displace less bone than the conventional bone screws, but are generallynot able to withstand high tensile loads without structural failure.European Patent No. 0 374 088 A1 discloses a bone screw having atwin-corkscrew design. In this twin-corkscrew design, which is formed bydrilling a passage up through a screw having a solid shank and thenmachining out the material between the two corkscrews, the junction ofthe corkscrews with the shank is unlikely to be capable of structurallywithstanding high tensile loads and repetitive fatigue loads. Thisstructural weakness in the design of the screw in the EP 0 374 088document is further compounded by the corkscrews having a larger overalldiameter than the head of the screw where torque is applied.

[0005] One of the more challenging applications of a bone screw isimplantation of the screw into the cancellous bone of a patient's spineor pelvis. For example, bone screws are frequently implanted into thecancellous bone of a patient's lumbar vertebrae during a spinal fixationprocedure to correct scoliosis. Once implanted, the bone screws are usedto mount suitable spinal fixation instrumentation, such as clamps, rods,and plates. Unfortunately, many of the known bone screws, such as thosedescribed above, can be susceptible to toggling in the vertebral bodyand can also pull out of the vertebral body due to the substantialforces on the screws from human body movement and muscle memory. Inorder to achieve a high pull-out resistance, it is known to thread abone screw all of the way through a vertebrae and place a nut on theopposite side. However, use of such a nut increases the complexity ofthe surgical procedure.

[0006] Hence, it is desirable to provide an apparatus for implantationinto a bone in a patient's body in a minimally invasive endoscopicprocedure, wherein the apparatus provides a platform for connectingspinal fixation instrumentation and, when implanted, is highly resistantto toggling in the bone and to being pulled out of the bone despite thesubstantial forces on the apparatus from human body movement and musclememory.

SUMMARY OF THE INVENTION

[0007] The present invention is an anchor which is implantable into abone in a patient's body and, when implanted, is resistant to togglingin the bone and to being pulled from the bone. The anchor comprises ahead end portion having a surface that extends transverse to a centralaxis of the anchor and is engagable with the bone. A plurality ofhelical spikes extend from the surface on the head end portion of theanchor and are engagable with the bone. Each of the plurality of helicalspikes has a helical central axis that forms a helix around the centralaxis of the anchor. Each of the plurality of helical spikes further hasa circular cross-sectional configuration as viewed in a plane extendingperpendicular to the helical central axis of each of the helical spikesand a distal end portion with a tip that penetrates the bone as the headend portion is rotated relative to the bone.

[0008] The present invention further provides a method of implanting ananchor in a patient's body. According to the inventive method, a cannulais positioned relative to a bone in a patient's body. An anchor, havinga plurality of helical spikes extending from a surface on a head endportion of the anchor, is moved along a path extending through thecannula with pointed end portions of the helical spikes leading and thehead end portion trailing. The bone is engaged with the pointed endportions of the helical spikes while moving the anchor along the pathextending through the cannula. The anchor is rotated about a centralaxis of the anchor after engaging the bone with the pointed end portionsof the helical spikes. The bone is penetrated with the helical spikes asthe anchor is rotated about its central axis. The surface on the headend portion of the anchor is moved into engagement with the bone at alocation between the helical spikes.

[0009] The present invention further provides an apparatus comprising atleast one anchor which is implantable into a bone in a patient's bodyand, when implanted, is resistant to toggling in the bone and to beingpulled from the bone, and a fixation implant for extending between andconnecting a plurality of bones. The at least one anchor includes a headend portion having a surface which extends transverse to a central axisof the anchor and is engagable with the bone. The head end portion hasmeans for connecting with the fixation implant. The head end portion ofthe at least one anchor has a plurality of helical spikes that extendfrom the surface and which extends transverse to a central axis of theat least one anchor and are engagable with the bone. Each of theplurality of helical spikes has a helical central axis that forms ahelix around the central axis of the at least one anchor. Each helicalspike of the plurality of helical spikes further has a distal endportion with a tip that penetrates the bone as the at least one anchoris rotated relative to the bone and a circular cross-sectionalconfiguration as viewed in a plane extending perpendicular to thehelical central axis of the one helical spike throughout a length of theone helical spike extending from the proximal end portion to the distalend portion of the one helical spike.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The foregoing and other features of the present invention willbecome apparent to those skilled in the art to which the presentinvention relates upon reading the following description with referenceto the accompanying drawings, in which:

[0011]FIG. 1 is a schematic anterior view of an apparatus constructed inaccordance with the present invention implanted in a vertebral body;

[0012]FIG. 1A is view similar to FIG. 1 showing the apparatus partiallyimplanted;

[0013]FIG. 2 is a schematic anterior view of several vertebral bodiesimplanted with the apparatus of FIG. 1 and connected by a spinalfixation implant in accordance with the present invention;

[0014]FIG. 3 is a side view taken along line 3-3 in FIG. 2;

[0015]FIG. 4 is an exploded perspective view of the apparatus of FIG. 2,and illustrates a driver for rotating the apparatus;

[0016]FIG. 5 is a side view of the apparatus of FIG. 1;

[0017]FIG. 6 is a sectional view taken along 6-6 in FIG. 5;

[0018]FIG. 6A is a view similar to FIG. 6 illustrating an alternativefeature of the invention;

[0019]FIG. 7 illustrates an alternate configuration for an end portionof the apparatus of FIG. 1;

[0020]FIG. 8 is a side view illustrating a second embodiment of anapparatus in accordance with the present invention;

[0021]FIG. 9 is a sectional view taken along line 9-9 in FIG. 8;

[0022]FIG. 10 is an exploded perspective view illustrating a thirdembodiment of an apparatus in accordance with the present invention;

[0023]FIG. 11 is a schematic side view of the apparatus of FIG. 10implanted in a pair of vertebral bodies;

[0024]FIG. 12 is a schematic view, partially in section, of a fourthembodiment of the present invention;

[0025]FIG. 12A is a view similar to FIG. 12 illustrating an alternateimplantation of the apparatus of FIG. 12;

[0026]FIG. 13 is an exploded perspective view of the apparatus of FIG.12;

[0027]FIG. 14 is a schematic posterior view illustrating a fifthembodiment of the present invention;

[0028]FIG. 15 is a side view of FIG. 14;

[0029]FIG. 16 is a sectional view taken along line 16-16 in FIG. 15;

[0030]FIG. 16A is a view similar to FIG. 16 illustrating an alternateconfiguration;

[0031]FIG. 17 is a side view of a starter tool for use with the presentinvention; and

[0032]FIG. 18 is a view showing the starter tool of FIG. 17.

DESCRIPTION OF EMBODIMENTS

[0033] The present invention is directed to an apparatus forimplantation into a bone in a patient's body, and is particularlydirected to an apparatus that, when implanted, is resistant to togglingin the bone and to being pulled from the bone. As representative of thepresent invention, FIG. 1 illustrates an apparatus 10 implanted in alumbar vertebrae 12. It should be understood that the apparatus 10 couldbe implanted into any number of different bones in a mammalian body,including but not limited to vertebral bones. The lumbar vertebrae 12has a concave side surface 14.

[0034] The apparatus 10 comprises an anchor 20 made from a biocompatiblematerial, such as titanium or stainless steel. It is contemplated thatthe biocompatible material used to make the anchor 20 could also bebiodegradable. The anchor 20 is centered about a longitudinal axis 22(FIG. 5). The anchor 20 includes a platform 24 having a generallycylindrical outer surface 26 extending between oppositely disposed firstand second ends 28 and 30 of the platform. The platform 24 includes agenerally rectangular slot 32 that extends axially from the first end 28toward the second end 30 of the platform. Adjacent the first end 28, theouter surface 26 of the platform 24 includes first and second segmentsof external threads 34 and 36 that are separated by the slot 32. Theslot 32 and the threads 34 and 36 provide structure for connectingspinal fixation instrumentation to the platform 24 as discussed furtherbelow. The second end 30 of the platform 24 includes an end surface 38(FIG. 6) having a convex shape that is complimentary to the shape of theconcave side surface 14 of the vertebrae 12. It is contemplated that thecomplimentary shape of the end surface 38 could range from concave toflat to convex depending on the specific shape of the bone surface to beengaged. The end surface 38 of the platform 24 may include barbs (notshown) or other suitable structure for fixedly engaging the side surface14 of the vertebrae 12. As shown in FIG. 6A, the end surface 38 may havea porous texture, formed by mesh, beads, or a coating such as a ceramiccoating, that increases the surface area of the end surface to promotebone in-growth and thus help with long term fixation of the anchor 20 tothe bone.

[0035] As shown in FIG. 1A, the platform 24 may further include acentral bore 39 for receiving a wire (or pin) 21 that has beenpreviously passed through the vertebrae 12 across the anatomicallydefined trajectory. The wire 21 is used to ensure proper alignment ofthe anchor 20 during insertion into the vertebrae.

[0036] First and second helical spikes 50 and 52 project tangentiallyfrom the end surface 38 of the platform 24. The helical spikes 50 and 52resemble a pair of intertwined corkscrews. According to the embodimentillustrated in FIGS. 1-6, the first and second helical spikes 50 and 52extend around the axis 22. The spikes 50 and 52 extend in a helicalpattern about the axis 22 at the same, constant radius R1. It iscontemplated, however, that the first and second helical spikes 50 and52 could extend about the axis 22 at different radiuses. Further, it iscontemplated that the radius of one or both of the first and secondhelical spikes 50 and 52 could increase or decrease as the helicalspikes extend away from the platform 24. In order for the anchor 20 tobe implanted endoscopically through a typical cannula 15 (FIG. 1A), theplatform 24 and the helical spikes 50 and 52 should be less than 20 mmin overall diameter. It should be understood that the anchor 20 couldhave an overall diameter that is greater than 20 mm for certainapplications, and that the anchor could be also implanted in an opensurgical procedure. However, for structural stability reasons, theoverall diameter of the helical spikes 50 and 52 should remain less thanor equal to the diameter of the platform 24.

[0037] In the illustrated embodiment of FIGS. 1-6, the first and secondhelical spikes 50 and 52 have the same axial length, and also have thesame circular cross-sectional shape. It is contemplated, however, thatthe first and second helical spikes 50 and 52 could have different axiallengths. Further, it is contemplated that the helical spikes 50 and 52could have a different cross-sectional shape, such as an oval shape. Italso contemplated that the first and second helical spikes 50 and 52could have different cross-sectional areas (i.e., one spike beingthicker than the other spike). Finally, it is contemplated that thehelical spikes 50 and 52 should have the same pitch, and that the pitchof the helical spikes would be selected based on the specific surgicalapplication and quality of the bone in which the anchor 20 is to beimplanted.

[0038] Each of the first and second helical spikes 50 and 52 can bedivided into three portions: a connecting portion 54, an intermediateportion 56, and a tip portion 58. The connecting portion 54 of each ofthe helical spikes 50 and 52 is located at a proximal end 60 thatadjoins the end surface 38 of the platform 24. The connection portion 54may include barbs (not shown) for resisting pull-out of the helicalspikes 50 and 52 from the vertebrae 12. According to one method formanufacturing the anchor 20, the connecting portion 54 of each of thehelical spikes 50 and 52 is fixedly attached to the platform 24 byinserting, in a tangential direction, the proximal ends 60 of thehelical spikes into openings (not shown) in the end surface 38 andwelding the connecting portions 54 to the platform. The insertedproximal ends 60 of the helical spikes 50 and 52 help to reduce tensilebending stresses on the helical spikes under tensile (or pull-out)loads.

[0039] Alternatively, the helical spikes 50 and 52 may be formedintegrally with the platform 24, such as by casting the anchor 20. Ifthe anchor 20 is cast, it is contemplated that a fillet (not shown) maybe added at the junction of the helical spikes 50 and 52 and theplatform 24 to strengthen the junction and minimize stressconcentrations at the connecting portions 54. The fillet at the junctionof the helical spikes 50 and 52 and the platform 24 also helps to reducebending stresses in the connection portions 54 of the helical spikesunder tensile (or pull-out) loads.

[0040] As best seen in FIG. 6, the connecting portions 54 at theproximal ends 60 of the first and second helical spikes 50 and 52 arespaced 180° apart about the axis 22 to balance the anchor 20 and evenlydistribute loads on the helical spikes. The connecting portion 54 ofeach of the helical spikes 50 and 52 has a first cross-sectionaldiameter D1 (FIG. 5).

[0041] The tip portion 58 of each of the helical spikes 50 and 52 islocated at a distal end 62 of the helical spikes. The intermediateportion 56 of each of the helical spikes 50 and 52 extends between thetip portion 58 and the connecting portion 54. The intermediate portion56 and the tip portion 58 of each of the helical spikes 50 and 52 has asecond cross-sectional diameter D2 that is less than or equal to thefirst cross-sectional diameter D1 of the connecting portions 54. If thesecond cross-sectional diameter D2 is less than the firstcross-sectional diameter D1, the increased thickness of the connectingportions 54 of the helical spikes 50 and 52 will help to provide theanchor 20 with increased tensile strength at the junction of the helicalspikes and the platform 24.

[0042] The tip portion 58 of each of the helical spikes 50 and 52illustrated in FIGS. 1-6 has an elongated conical shape with a sharppointed tip 68 for penetrating into the vertebrae 12 as the platform 24of the anchor 20 is rotated in a clockwise direction. FIG. 7 illustratesan alternative, self-tapping configuration for the tip portions 58 whichincludes a planar surface 66 for driving into the vertebrae 12, in thesame manner that a wood chisel turned upside-down drives into wood, asthe platform 24 is rotated. It is contemplated that the tip portions 58could also have a pyramid shape (not shown), similar to the tip of anail.

[0043] To implant the anchor 20, a starter tool 600 (FIG. 17) is used topunch two holes 602 and 604 (FIG. 18) in the cortical bone of thevertebrae 12. The starter tool 600 includes a platform 624 similar tothe platform 24 and a plurality of helical spikes 650 and 652 similar tothe helical spikes 50 and 52. The platform 624 includes a feature, suchas a hexagonal drive projection 630, for drivingly rotating the startertool 600. The spikes 650 and 652 correspond in diameter and quantity tothe helical spikes 50 and 52, but are much shorter in axial length inorder to increase their strength and resistance to radially outwarddeformation. The holes 602 and 604 are punched in locations thatcorrespond to the spacing of the tip portions 58 of the helical spikes50 and 52 on the anchor 20. It should be noted that one or both of theconfigurations of the tip portions 58 illustrated in FIGS. 1-7 may beable to punch through the cortical bone upon rotation of the anchor 20,thus eliminating the need for the starter tool 600 to punch holes in thecortical bone.

[0044] As shown in FIG. 18, alignment of the starter tool 600 along thedesired axis through the vertebrae 12 may be ensured by threading thestarter tool down over a wire 21 that has been previously passed throughthe vertebrae. To allow for this, the starter tool 600 may optionallyinclude a central bore 660.

[0045] The tip portions 58 are then placed in the holes 602 and 604 inthe vertebrae 12 and a rotatable driver 70 (FIG. 4) is inserted into theslot 32 in the platform 24. The driver 70 is then rotated, causing theanchor 20 to rotate as well. It is contemplated that a cylindricalsleeve 17 (FIG. 1A) may be placed around the intermediate portions 56and the connecting portions 54 of the helical spikes 50 and 52 toprevent the helical spikes from deforming radially outward during theinitial rotation of the anchor 20.

[0046] Rotation of the anchor 20 screws the helical spikes 50 and 52into the cancellous bone of the vertebrae 12. The tangentially-orientedconnection between the connecting portions 54 of the helical spikes 50and 52 and the platform 24 minimizes bending loads on the connectingportions during rotation of the anchor 20. Further, thetangentially-oriented connection ensures that the force vector resultingfrom torque and axial force applied by the driver 70 to platform 24 istransmitted along the helical centerline (not shown) of each of thehelical spikes 50 and 52.

[0047] As the anchor 20 is rotated, the tip portion 58 of the firsthelical spike 50 penetrates the cancellous bone and cuts a first helicaltunnel 80 (FIG. 1) through the vertebrae 12. Simultaneously, the tipportion 58 of the second helical spike 52 penetrates the cancellous boneof the vertebrae 12 and cuts a second helical tunnel 82. The first andsecond helical tunnels 80 and 82 are shaped like the helical spikes 50and 52, respectively. Continued rotation of the anchor 20 embeds thehelical spikes 50 and 52 deeper into the cancellous bone of thevertebrae 12. The anchor 20 is rotated until the convex end surface 38of the platform 24 seats against the concave side surface 14 of thevertebrae 12 as shown in FIG. 1.

[0048]FIGS. 2 and 3 illustrate how the anchor 20 is used for segmentalspinal fixation of lumbar vertebrae to treat a patient with scoliosis.Lumbar vertebrae L3-L5, indicated by reference numbers 90, 91, and 92,respectively, are shown in FIGS. 2 and 3. Normally, disk material 94separates each of the lumbar vertebrae 90-92. However, in order tocorrect the scoliosis, the surgeon removes the disk material 94 betweenthe vertebrae 90-92. The spaces left between the vertebrae 90-92 aresubsequently filled with bone graft material 96 (shown schematically inFIGS. 2 and 3) that fuses the vertebrae together over time. Spinalfixation instrumentation, such as a rod or a beam 100, is used tosupport the vertebrae 90-92 until the vertebrae fuse together.

[0049] As shown in FIGS. 2 and 3, the vertebrae 90-92 are each implantedwith the anchor 20 according to the present invention as describedabove. The beam 100, which is bent into a desired shape by the surgeon,is placed into the slot 32 in each of the anchors 20. A nut 102 is thenscrewed onto the threads 34 and 36 on each of the platforms 24 and istightened to secure the beam 100 to each of the anchors 20.

[0050] When implanted, the anchors 20 are subjected to substantialforces caused by human body movement and muscle memory. In some cases,these forces can tend to pull the known screws used in such anapplication out of the vertebrae 90-92 or can cause the screws to togglein the vertebrae. However, when the helical spike 50 and 52 are embeddedin the vertebrae 90-92, the two helical spikes of the anchors 20 providethe anchors with a high resistance to pull-out forces. Preliminarycadaver testing indicates that the anchor 20 is so resistant to beingpulled axially from a vertebral body that the vertebral body itself islikely to fail before the anchor pulls out under high tensile load.Further, the helical spikes 50 and 52, and their tangential connectionwith the platform 24, provide the anchors 20 with a high resistance totoggling in the vertebrae 90-92.

[0051]FIGS. 8 and 9 illustrate an apparatus 210 constructed inaccordance with a second embodiment of the present invention. In thesecond embodiment of FIGS. 8 and 9, reference numbers that are the sameas those used in the first embodiment of FIGS. 1-6 designate parts thatare the same as parts in the first embodiment.

[0052] According to the second embodiment, the apparatus 210 comprisesan anchor 220 having three helical spikes 230, 231, and 232 projectingtangentially from the end surface 38 of the platform 24. The spikes230-232 extend around the axis 22. As shown in FIG. 9, the connectingportions 54 at the proximal ends 60 of the helical spikes 230-232 arespaced 120° apart about the axis 22, which balances the anchor 220 andevenly distributes loads on the helical spikes. As in the firstembodiment of FIGS. 1-6, in the second embodiment of FIGS. 8 and 9, thecross-sectional diameter of the connecting portions 54 of the helicalspikes 230-232 is greater than or equal to the cross-sectional diameterof the intermediate portions 56 and the tip portions 58 of the helicalspikes.

[0053] Each of the three helical spikes 230-232 extends in a helicalpattern about the axis 22 at the same, constant radius R1. It iscontemplated, however, that one or more of the helical spikes 230-232could extend about the axis 22 at different radiuses. Further, it iscontemplated that the radius of one or more helical spikes 230-232 couldincrease or decrease as the helical spikes extend away from the platform24.

[0054] As shown in FIG. 8, the three helical spikes 230-232 have thesame axial length and also have the same circular cross-sectional shape.It is contemplated, however, that one or more of the helical spikes230-232 could have different axial lengths. Further, it is contemplatedthat one or more of the helical spikes 230-232 could have a differentcross-sectional shape, such as an oval shape. It also contemplated thatthe one or more of the helical spikes 230-232 could have differentcross-sectional areas (i.e., one spike being thicker or thinner than theother two spikes). Finally, it is contemplated that the helical spikes230-232 should have the same pitch, and that the pitch of the helicalspikes would be selected based on the specific surgical application andquality of the bone in which the anchor 20 is to be implanted.

[0055] The tip portion 58 of each of the helical spikes 230-232illustrated in FIG. 8 has an elongated conical shape for penetratinginto a vertebrae as the platform 24 of the anchor 220 is rotated in theclockwise direction. It should be understood that the tip portions 58 ofthe helical spikes 230-232 of the anchor 220 could alternatively beconfigured like the tip portions illustrated in FIG. 7.

[0056] The anchor 220 according to the second embodiment of FIGS. 8 and9 is implanted in a vertebrae in the same manner as the anchor 20according to the first embodiment. Further, the anchor 220 according tothe second embodiment may also be used to mount spinal fixationinstrumentation in same manner as the anchor 20 according to the firstembodiment. The anchor 220 according to the second embodiment, whenimplanted in a vertebrae, is highly resistant to being pulled out of thevertebrae and to toggling in the vertebrae despite being subjected tosubstantial forces caused by human body movement and muscle memory.

[0057]FIGS. 10 and 11 illustrate an apparatus 310 constructed inaccordance with a third embodiment of the present invention. In thethird embodiment of FIGS. 10 and 11, reference numbers that are the sameas those used in the first embodiment of FIGS. 1-6 designate parts thatare the same as parts in the first embodiment.

[0058] According to the third embodiment, the apparatus 310 comprises ananchor 320 having a platform 324. The platform 324 has a threaded outersurface 330 adjacent a first end portion 332 and a cylindrical outersurface 340 adjacent a second end portion 342. The first end portion 332of the platform 324 further includes an axial recess 334. The recess 334has a hexagonal configuration for receiving a tool (not shown) fordrivingly rotating the anchor 320. The first and second helical spikes50 and 52 project from the end surface 38 of the platform 324.

[0059] The apparatus 310 further includes a plate 350 and a nut 360. Theplate 350 has a first opening 352 for receiving the portion of theplatform 324 which has the threaded outer surface 330. The plate 350 hasa second opening 354 for receiving a second anchor 320 (see FIG. 11) orother fixation instrumentation (not shown). When the anchor 320 isimplanted in a vertebrae, the nut 360 screws onto the threaded outersurface 330 of the platform 324 to secure the plate 350 to the anchor320.

[0060] The anchor 320 according to the third embodiment of FIGS. 10 and11 is implanted in a vertebrae in the same manner as the anchor 20according to the first embodiment. FIG. 11 shows a pair of the anchors320 implanted in two cervical vertebrae 370 and 380. The end surface 38of each of the anchors 320 engages a respective anterior surface on eachof the vertebrae 370 and 380. The plate 350 connects the anchors 320 tohelp support the vertebrae 370 and 380 and transfer loads between thevertebrae until the bone graft material 96 fuses the vertebrae. Like theanchor 20 according to the first embodiment, the anchor 320 according tothe third embodiment, when implanted in the vertebrae, is highlyresistant to being pulled out of the vertebrae and to toggling in thevertebrae despite being subjected to substantial forces caused by humanbody movement and muscle memory.

[0061]FIGS. 12 and 13 illustrate an apparatus 410 constructed inaccordance with a fourth embodiment of the present invention. In thefourth embodiment of FIGS. 12 and 13, reference numbers that are thesame as those used in the first embodiment of FIGS. 1-6 designate partsthat are the same as parts in the first embodiment.

[0062] According to the fourth embodiment, the apparatus 410 comprises apair of anchors 420 extending around a longitudinal axis 422. Each ofthe anchors 420 includes a platform 424 that is substantially wider thanthe platform 24 of the anchor 20 in the first embodiment. The platform424 has a cylindrical outer surface 426 that extends between oppositelydisposed first and second end surfaces 428 and 430. An attachment tab440 projects axially away from the first end surface 428 of the platform424. The attachment tab 440 includes a pair of oppositely disposedplanar surfaces 442 and a pair of oppositely disposed arcuate surfaces444.

[0063] The attachment tabs 440 provide structure for connecting spinalfixation instrumentation to each of the platforms 424 and for drivingthe anchors 420. The second end surface 430 of the platform 424 of eachanchor 420 has a shape that is complimentary to the shape of an upper orlower surface of a vertebrae. Similar to the first embodiment of FIGS.1-6, the anchors 420 have first and second helical spikes 450 and 452that project from the second end surface 430 of the platform 424. Thehelical spikes 450 and 452 extend along the axis 422, but aresignificantly larger in diameter than the helical spikes 50 and 52 inthe first embodiment. It should be understood that the anchors 420 couldalternatively have three helical spikes as shown in the secondembodiment of FIGS. 8 and 9.

[0064] The apparatus 410 according to the fourth embodiment of FIGS. 12and 13 is particularly useful for a corpectomy application in which adamaged vertebrae is removed. As is shown in FIG. 12, after a portion ofa damaged vertebrae 460 is removed, a first one of the pair of anchors420 is implanted into a vertebrae 462 directly above the removedvertebrae 460 and a second one of the pair of anchors 420 is implantedinto a vertebrae 464 directly below the removed vertebrae.

[0065] The anchors 420 are implanted in the vertebrae 462 and 464 inmuch the same manner as the anchor 20 according to the first embodiment.A rotatable tool (not shown) engages the planar surfaces 442 on theattachment tab 440 and rotates each of the anchors 420 to screw thehelical spikes 450 and 452 of each of the anchors into the respectivevertebrae 462 and 464. The anchors 420 are implanted so that they extendco-linearly along the axis 422. When implanted, the helical spikes 450and 452 of the anchor 420 in the vertebrae 462 extend in an upwarddirection from the platform 430 of the upper (as viewed in the Figures)anchor, while the helical spikes 450 and 452 of the other anchor in thevertebrae 464 extend in a downward direction from the platform 430 ofthe lower (as viewed in the Figures) anchor.

[0066] A spinal fixation implant in the form of a cylinder member 480connects the pair of anchors 420 to structurally support the vertebralcolumn in the absence of the removed vertebrae 460. The cylinder member480 has a cylindrical outer surface 482 and an eccentric inner surface484. The cylinder member 480 has a first slot 486 at a first end 488 anda second slot 490 at a second end 492. The first and second slots 486and 490 receive the attachment tabs 440 on the anchors 420 and allow thecylinder member 480 to be inserted between the anchors. Once insertedbetween the anchors 420, the cylinder member 480 is then rotatedrelative to the anchors about the axis 422. Rotation of the cylindermember 480 brings the arcuate surfaces 444 on the attachment tabs 440 ofthe anchors 420 into frictional engagement with the eccentric innersurface 484 of the cylinder member, thereby securing the cylindermember.

[0067] As with the previous embodiments, the anchors 420 according tothe fourth embodiment, when implanted, are highly resistant to beingpulled out of the vertebrae 462 and 464 and to toggling in the vertebraedespite being subjected to substantial forces caused by human bodymovement and muscle memory.

[0068]FIG. 12A illustrates an alternate implantation of the apparatus410 of FIG. 12. In FIG. 12A, an apparatus 410′ comprises a pair ofanchors 420′ that are identical to the anchors 420 described above,except that they include a platform 424′ that has a porous surface 430′,as has been previously described. The porous surface 430′ has anincreased surface area that promotes bone in-growth. In all otherrespects, the apparatus 410′ is identical to the apparatus 410 describedabove.

[0069] The apparatus 410′ is implanted in the vertebrae 462 and 464 inthe same manner as described above, except that the platforms 424′ onthe anchors 420′ are recessed into the end surfaces of the twovertebrae, as may be seen in FIG. 12A. The porous surfaces 430′ on theplatforms 424′ help with long term fixation of the anchors 420′ to thevertebrae 462 and 464.

[0070] FIGS. 14-16 illustrate an apparatus 510 constructed in accordancewith a fifth embodiment of the present invention. In the fifthembodiment of FIGS. 14-16, reference numbers that are the same as thoseused in the first embodiment of FIGS. 1-6 designate parts that are thesame as parts in the first embodiment.

[0071] According to the fifth embodiment, the apparatus 510 comprises ananchor 520 implanted into a sacrum 540. The anchor 520 includes aplatform 524 having a generally cylindrical outer surface 526 extendingbetween oppositely disposed first and second ends 528 and 530. Theplatform 524 includes a slot 532 that extends axially from the first end528 toward the second end 530 of the platform. Adjacent the first end528, the outer surface of the platform 524 includes first and secondsegments of external threads 534 and 536 that are separated by the slot532. The slot 532 and the threads 534 and 536 on the platform 524provide structure for connecting a rod 550 to the anchor 520.

[0072] The second end 530 of the platform 524 includes an end surface542 having a shape that is a complimentary to the shape of a surface 544(FIG. 16) of the sacrum 540. The anchor 520 includes the first andsecond helical spikes 50 and 52 that extend from the end surface 542 ofthe platform 524.

[0073] The anchor 520 according to the fifth embodiment of FIGS. 14-16is implanted in the sacrum 540 in much the same manner as the anchor 20according to the first embodiment is implanted in the vertebrae 12. Asshown in FIG. 15, in addition to the anchor 520 being implanted in thesacrum 540, known screws 560 are implanted in the pedicles of lumbarvertebrae 562 and 564 above the sacrum. The rod 550 is then bent into adesired shape by the surgeon and placed into the slot 532 in theplatform 524 of the anchor 520. A seat 570 is placed over the first end528 of the platform 524 and engages the rod 550. A nut 572 screws downover the seat 570 and clamps the rod 550 to the anchor 520. In a similarfashion, the nuts 580 secure the rod 550 to the screws 560 implanted inthe vertebrae 562 and 564 above the sacrum 540.

[0074] As in the first embodiment, the anchor 520 according to the fifthembodiment, when implanted, is highly resistant to being pulled out ofthe sacrum 540 and to toggling in the sacrum despite being subjected tosubstantial forces caused by human body movement and muscle memory.

[0075]FIG. 16A illustrates an alternate construction for an anchor 520′in which the surface 542′ has a porous texture, such as has beenpreviously described in connection with FIG. 6A. Further, the platform524 on the anchor 520′ includes a cylindrical outer surface 580 thatextends parallel to the axis of the anchor and has external threads 582for screwing into the bone. The anchor 520′ is thus designed to berecessed into the bone as shown in FIG. 16A to help with fixation as thebone in-growth occurs.

[0076] From the above description of the invention, those skilled in theart will perceive improvements, changes and modifications. It should beunderstood that the present invention can be used for a variety ofpurposes and implanted in other bones besides bones in the vertebralcolumn. Further, it should be understood that more than one of theapparatuses disclosed herein may be implanted into a single bone, suchas a vertebral body. Such improvements, changes and modifications withinthe skill of the art are intended to be covered by the appended claims.

Having described the invention, I claim:
 1. An anchor which isimplantable into a bone in a patient's body and, when implanted, isresistant to toggling in the bone and to being pulled from the bone,said anchor comprising: a head end portion having a surface that extendstransverse to a central axis of said anchor and is engagable with thebone; and a plurality of helical spikes extending from said surface onsaid head end portion of said anchor and are engagable with the bone,each of said plurality of helical spikes having a helical central axisthat forms a helix around said central axis of said anchor, each of saidplurality of helical spikes further having a circular cross-sectionalconfiguration as viewed in a plane extending perpendicular to saidhelical central axis of each of said helical spikes and having a distalend portion with a tip that penetrates the bone as said head end portionis rotated relative to the bone.
 2. An anchor as set forth in claim 1wherein each helical spike of said plurality of helical spikes is formedseparately from said head end portion of said anchor and has a proximalend portion which extends through said surface on said head end portionof said anchor into an opening in said head end portion of said screw.3. An anchor as set forth in claim 2 wherein said proximal end portionof each helical spike extends at an acute angle to said surface on saidhead end portion of said anchor.
 4. An anchor as set forth in claim 3wherein said surface on said head end portion of said anchor has aconvex configuration and has a center of curvature which is offset fromsaid surface on said head end portion of said anchor in a direction awayfrom said distal end portion of each of said plurality of helicalspikes.
 5. An anchor as set forth in claim 1 wherein each of saidhelical spikes has a circular cross-sectional configuration with thesame diameter throughout the extent of each of said helical spikesbetween a proximal end portion of each of said helical spikes and saiddistal end portion of each of said helical spikes.
 6. An anchor as setforth in claim 1 wherein said head end portion includes means forconnection with a spinal fixation implant.
 7. An anchor as set forth inclaim 1 wherein said plurality of helical spikes includes at least twohelical spikes.
 8. An anchor as set forth in claim 1 wherein saidplurality of helical spikes includes at least three helical spikes. 9.An anchor as set forth in claim 1 wherein said surface on said head endportion is circular and is disposed in a coaxial relationship with saidcentral axis of said anchor and has a diameter which is greater than anoutside diameter of said helical spikes.
 10. An anchor as set forth inclaim 1 wherein said surface on said head end portion has a rough,porous texture that provides an increased surface area for said surfaceto promote bone in-growth.
 11. An anchor as set forth in claim 1 whereinsaid head end portion includes a cylindrical outer surface extendingparallel to said central axis and having external threads for screwinginto the bone.
 12. A method of implanting an anchor in a patient's body,said method comprising the steps of: positioning a cannula relative to abone in a patient's body; moving an anchor having a plurality of helicalspikes extending from a surface on a head end portion of the anchoralong a path extending through the cannula with pointed end portions ofthe helical spikes leading and the head end portion trailing; engagingthe bone with the pointed end portions of the helical spikes whilemoving the anchor along the path extending through the cannula; rotatingthe anchor about a central axis of the anchor after engaging the bonewith the pointed end portions of the helical spikes; penetrating thebone with the helical spikes as the anchor is rotated about its centralaxis; and moving the surface on the head end portion of the anchor intoengagement with the bone at a location between the helical spikes.
 13. Amethod as set forth in claim 12 further including the step of forming aplurality of holes in the bone, said step of engaging the bone with thepointed end portions of the helical spikes includes moving the pointedend portion of each helical spike in the plurality of helical spikesinto one of the plurality of holes in the bone.
 14. A method as setforth in claim 13 further including the step of providing an anchorstarter having a plurality of helical spikes that correspond in quantityand size to the plurality of helical spikes on said anchor, theplurality of helical spikes on the anchor starter having a short axiallength to resist radially outward deformation, said step of forming aplurality of holes includes rotating the anchor starter to form theholes.
 15. A method as set forth in claim 14 further comprising thesteps of: positioning a wire through the bone along a desired axis;placing the anchor starter over the wire and sliding the anchor startertoward the bone along the desired axis; engaging the surface of the bonewith the helical spikes on the anchor starter and rotating the anchorstarter to form the holes; removing the anchor starter from the wire;placing the anchor over the wire and sliding the anchor toward the bonealong the desired axis; and inserting the helical spikes on the anchorinto the holes in the bone formed by the anchor starter.
 16. A method asset forth in claim 14 further comprising the steps of: positioning awire through the bone along a desired axis; and placing the anchor overthe wire and sliding the anchor toward the bone along the desired axis.17. A method as set forth in claim 12 further including the step oflimiting radially outward deformation of the helical spikes bypositioning a sleeve around the helical spikes during rotation of theanchor about a central axis of the anchor.
 18. An apparatus comprising:at least one anchor which is implantable into a bone in a patient's bodyand, when implanted, is resistant to toggling in the bone and to beingpulled from the bone; and a fixation implant for extending between andconnecting a plurality of bones; said at least one anchor including ahead end portion having a surface that extends transverse to a centralaxis of said anchor and is engagable with the bone, said head endportion having means for connecting with said fixation implant; saidhead end portion of said at least one anchor having a plurality ofhelical spikes that extend from said surface and which extend transverseto a central axis of said at least one anchor and are engagable with thebone, each of said plurality of helical spikes having a helical centralaxis that forms a helix around said central axis of said at least oneanchor; each helical spike of said plurality of helical spikes furtherhaving a distal end portion with a tip which penetrates the bone as saidat least one anchor is rotated relative to the bone and a circularcross-sectional configuration as viewed in a plane extendingperpendicular to said helical central axis of said one helical spikethroughout a length of said one helical spike extending from saidproximal end portion to said distal end portion of said one helicalspike.
 19. An apparatus as set forth in claim 18 wherein said proximalend portion of each of said plurality of helical spikes extend at anacute angle to said surface on said head end portion of said at leastone anchor.
 20. An apparatus as set forth in claim 18 wherein saidsurface on said head end portion of said at least one anchor has aconvex configuration and has a center of curvature which is offset fromsaid surface on said head end portion in a direction away from saiddistal end portion of each of said plurality of helical spikes.
 21. Anapparatus as set forth in claim 18 wherein said surface on said head endportion is circular and is disposed in a coaxial relationship with saidcentral axis of said at least one anchor, said head end portion having adiameter which is greater than an outside diameter of said plurality ofhelical spikes.
 22. An apparatus as set forth in claim 18 wherein saidplurality of helical spikes includes at least two helical spikes.
 23. Anapparatus as set forth in claim 18 wherein said plurality of helicalspikes includes at least three helical spikes.
 24. An apparatus as setforth in claim 18 wherein said surface on said head end portion has arough, porous texture that provides an increased surface area to promotebone in-growth.
 25. An apparatus as set forth in claim 18 wherein saidat least one anchor comprises a first anchor for implantation into afirst bone and a second anchor for implantation into a second bone. 26.An apparatus as set forth in claim 18 further comprising an anchorstarter for forming starting holes in the bone that said plurality ofhelical spikes are received in, said anchor starter comprising aplatform having a surface that extends transverse to a longitudinal axisof said anchor starter and a plurality of helical spikes extending fromsaid surface, said plurality of helical spikes on said anchor startercorresponding in quantity and size to said plurality of helical spikeson said anchor but are substantially shorter in axial length to resistradially outward deformation during rotation of said platform.
 27. Anapparatus as set forth in claim 18 wherein said head end portion of saidanchor includes a cylindrical outer surface extending parallel to saidcentral axis and having external threads for screwing into the bone.